Relative size of avian breeding territories and the risk of cuckoldry

Abstract

ANDERS PAPE MOLLER Department of Zoology, Uppsala University, Box 561, S-751 22 Uppsala, Sweden (Received3 July 1991; accepted 5 July 1991; MS. number. sc-670) As a student I was brought up with the truism that the function of breeding territories in birds was to provide an exclusive food source for successful reproduction. Later discussions with many colleagues left me with the impression that this was a well established fact. However, when I started to do field work on colour-ringed birds I very soon detected that most of the intruders on breeding territories were mated males from neigh- bouring territories apparently seeking extra-pair copulations with the resident female (Moller 1985). A subsequent review of the extensive literature revealed that this was a common pattern; most intrusions did occur during the presumed fertile period of the resident female (Maller 1987). However, this information had been totally neg- lected under the ruling paradigm that the primary function of breeding territories in birds was the defence of an exclusive food source. If most territorial intrusions occur during a particular time of the breeding cycle, I predicted that male birds should show a range of adaptations which would attempt to alleviate the costs of such intrusions (i.e. risks of cuckoldry). These adap- tations could be adjustments in the size of the breeding territory (Moller 1990), the rate of terri- torial patrols (Meller 1987), the rate of singing (Maller 1991) and breeding cycle levels of testoster- one (Moore t984; Birkhead & Moller 1992). Reviews of the extensive literature revealed that males of many birds do demonstrate a peak in song activity during the fertile period of their mate (Moiler 1991) and that the size of the territory changes in relation to the intrusion pressure (Moller 1990). These results suggested to me that alternative explanations for the functional signifi- cance of avian breeding territoriality had not been critically assessed. Dunn (1992) has suggested that the evidence in favour of the idea that male birds adjust the size of their territory to the risk of cuckoldry was at best equivocal. I will address each of his points in the following paragraphs. First, I do not think that it is generally possible to assess critically a hypothesis by means of a review. Well designed experiments are needed to evaluate critical assumptions and predictions. My paper on the size of avian breeding territories in relation to the stage in the breeding cycle should therefore only be considered a possible explanation for a long-neglected common pattern. Second, Dunn suggests that the general peak in territory size during the nest-building and egg- laying periods is consistent with three other hypotheses (the nest site, the male competition and the polygyny hypotheses). If nest sites are the limiting resource defended by males, one would predict that territory size would peak before a nest site is chosen. Dunn claims that the general peak in territory size during pair formation is consistent with this prediction. However, since most species do not show a peak in territory size during pair formation, but later during nest building and egg laying, I would rather suggest that this refutes the prediction because a nest site has already been chosen, and there would be no further need for defending a relatively large territory. Furthermore, male birds often adjust the location of their terri- tory without changing its size, when a nest site has been chosen by the female, particularly if the nest site is at the periphery or outside the territory (per- sonal observations on yellowhammers, Emberiza citrinella; J. Sundberg, personal communication). This suggests that the provisioning of nest sites is not the primary function of the territory. Dunn also suggests that the breeding cycle pattern of territory size is consistent with a male competition hypothesis, if he assumes that intruder pressure is relatively low at the beginning of the breeding season. If one also assumes an inverse relationship between territory size and intruder pressure, then territory size should decrease during the breeding cycle. I would suggest that a direct relationship between territory size and intruder pressure is more likely. Dunn concludes that it is impossible to test the male competition hypothesis without data on the pattern of intruder pressure. However, a review of data on the timing of in- trusions has already been presented, and data are 0003-3472/92/050860 + 02 $03.00/0 9 1992 The Association for the Study of Animal Behaviour 860